builder: run make single-threaded on windows
builder: run make single-threaded on windows

Will still honor MAKEFLAGS environment variable if set.

R=golang-dev
CC=bradfitz, golang-dev
http://codereview.appspot.com/4644049

wesolows/golang,wesolows/golang,wesolows/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,firebitsbr/golang-old,firebitsbr/golang-old,firebitsbr/golang-old,wesolows/golang,firebitsbr/golang-old,wesolows/golang-old,wesolows/golang,wesolows/golang-old,wesolows/golang-old,firebitsbr/golang-old